The present invention relates to a process for preparing polytetrafluoroethylene granular powder, more particularly to a process for preparing polytetrafluoroethylene granular powder having a sharp size distribution and an improved flowability.
In general, a polytetrafluoroethylene granular powder is required (1) to have excellent flowability, (2) to be soft and to be pressed by a comparative low pressure to form a dense preformed article, and (3) to have a large bulk density, since it is molded by a manner similar to that in powder metallurgy differing from the molding method for another thermoplastic resins such as melt extrusion, injection or compression molding.
For the purpose of preparing a polytetrafluoroethylene granular powder satisfying the above-mentioned requirements, there have been proposed various processes for preparing the granular powder consisting of agglomerate composed of primary particles, which has a high bulk density, an excellent flowability and a softness the same as in the primary particle, by agglomerating a polytetrafluoroethylene powder which has been obtained by pulverizing the raw polytetrafluoroethylene powder prepared by suspension-polymerization of tetrafluoroethylene. For instance, U.S. Pat. No. 3,265,679 describes a process which comprises wetting a polytetrafluoroethylene powder with an organic liquid being capable of wetting the powder and subjecting to agitation of the wetted powder to give granules having a dry sieve size in the range of 300 to 3000.mu.. Also British Pat. No. 1,100,388 and U.S. Pat. No. 3,527,857 describe a process for agglomerating a polytetrafluoroethylene powder by agitating the same in an aqueous medium containing an organic liquid capable of wetting the powder. According to the process described in British Pat. No. 1,100,388, there can be obtained a polytetrafluoroethylene granular powder consisting of secondary agglomerates composed of primary particles of particle size of less than 200.mu., having a particle size in the range of 100 to 5000.mu., a bulk density of more than 400 g./liter and an excellent flowability compared with conventional ones and giving a dense molded article, which process comprises agitating in water polytetrafluoro ethylene powder of a particle size of 1 to 200.mu. after wetting with a water-insoluble organic liquid having a surface tension of at most 35 dynes/cm. The thus obtained polytetrafluoroethylene granular powder has merits that the flowability of the powder is superior and the aggregation of the powder as a whole hardly occurs on storage and transferring or handling since the particle is spherical or nearly spherical and its outer surface is extremely smooth. However, according to this process, the obtained granular powder has a tendency that its particle size is large and not uniform, moreover contains extremely large particles as large as 5000.mu.. In case of molding from such a granular powder containing extremely large particles, the surface of molded article becomes uneven and, for instance, the surface grinding of the molded article is required for making the surface smooth when it is employed as a packing.
According to the investigations by the present inventors, it has been found that a polytetrafluoroethylene granular powder must possess the following properties in addition to the properties described in the above-mentioned British Pat. No. 1,100,388.
(a) In consideration of the fact that the smaller particle size of granular powder, the better mechanical properties and smoothness of a molded article, it has been found out that the upper limit of average particle size is 500.mu.. In particular, it is necessary for obtaining a molded article of small size with smooth surface that the average particle size of granular powder be at most 500.mu.. However, the smaller the particle size, the poorer flowability and non-aggregating property. Therefore, as a matter of course, a granular powder has an under limit in the particle size, and it has been found out that the under limit of the average particle size is about 100.mu..
(b) In case the powder contains extremely large particles, even if the average particle size of the granular powder falls in the range of 100 to 500.mu., surface of molded artilce does not become even. Also in case the powder containes extremely small particles, the flowability of granular powder is decreased. In view of this point, it has been found that at least 90% by weight of a whole granular powder must have a particle size of at most 1000.mu. and moreover at least 60% by weight of a whole granular powder must have a particle size in the range of 0.7 to 1.3 times its average particle size, especially 0.75 to 1.25 times.
(c) It is required that the powder flowability, an easiness of flowing of a granular powder, which is defined hereinafter, must be at least 3. In case the powder flowability of a granular powder is less than 3, it is difficult to charge the powder into a hollow of a mold for a short period of time particularly when using it in automatic molding.
(d) It is required that the surface roughness indicating a degree of smoothness of a molded article obtained from a granular powder, which is defined hereinafter, must be at most 2.0, especially at most 1.5.
On the other hand, it has been apparent that a granular powder satisfying the above properties cannot be obtained by the process of British Pat. No. 1,100,388 on industrial scale.
According to the process of British Pat. No. 1,100,388, a granular powder satisfying the above conditions may be prepared when a small amount of polytetrafluoroethylene powder is treated by a high speed agitation in a comparatively small-scale apparatus, but this process cannot be utilized on industrial scale production since an extremely high speed agitation and a considerable power are required. Moreover when an apparatus is large-scale, the amount of extremely large particles and extremely small particles in the obtained granular powder is increased and a granular powder having a uniform particle size cannot be obtained since continuously a uniform power could be hardly provided as a whole powder even if the agitation speed is increased. As a result, a size distribution of an obtained granular powder does not fall in the above-mentioned range and a flowability is decreased and a surface roughness of a molded article obtained from a granular powder becomes poor.
Further, according to the process by agitating polytetrafluoroethylene powder in water containing organic liquid, a particle size of obtained granular powder may be decreased by employing the least amount of organic liquid.
However, in that case, the surface of each particle does not become sufficiently smooth and the flowability of the obtained granular powder is poor so that it cannot be employed for an automatic molding, and the granular powder does not satisfy the above-mentioned properties.
U.S. Pat. No. 3,527,857 describes that agglomerated powder having a relatively small particle size which satisfies the property (a) among the above-mentioned properties (a) to (d) is obtained by this process, but according to the investigation by the present inventors, it has been found that this powder does not also satisfy the above-mentioned property (b), and as a result, the powder flowability of this powder is very poor. Although this patent describes that the agglomerated powder having a relatively small particle size is obtained, it is considered that this result is produced by the use of a very large agitation evergy and in this point the process of this patent is undesirable. Moreover, when the amount of the treated powder in one treatment is scaled up to a degree of industrially practicing the process, there is required a larger agitation energy which is not industrially acceptable.
Further, according to the process of U.S. Pat. No. 3,265,679 which comprises by mechanically agitating a finely divided polytetrafluoroethylene powder in the presence of only an organic liquid without using water as stated before, the obtained powder does not satisfy all of the above-mentioned properties (a) to (d).